The present invention relates to a multi-layer circuit board which comprises at least a first board layer comprising a lower surface and an upper surface, and at least a second board layer comprising a lower surface and an upper surface, and said multi-layer circuit board embedding a coaxial conductor comprising an inner conductor, an outer conductor enclosing the inner conductor at least partly, as well as a dielectric placed between said inner conductor and outer conductor.
The selection of various transmission lines for different uses for transferring electric signals from one place to another depends on the properties of the transmission lines, such as attenuation caused by the conductors, frequency band of the signal to be transferred, power capacity and size of the conductors. Compared with other known transmission lines, coaxial conductors have the advantage that they are suitable for broadband radio-frequency signals, even microwave-frequency signals. Other transmission lines include a dielectrically isolated wire, a twin wire, and a conductor comprising several twin wires. A special advantage of a coaxial conductor is excellent interference suppression properties due to its structure. Thus, electromagnetic fields outside the coaxial conductor disturb the signals to be transferred in the coaxial conductor less than in other transmission lines, in which such fields induce interfering signals.
According to prior art, wireless communication devices, such as mobile phones, use a coaxial conductor formed of a coaxial conductor for the transmission of a radio-frequency signal from components placed on a circuit board, such as integrated circuits (IC), to an antenna. A coaxial cable can be used also for the transmission of sensitive signals, such as audio signals, on a circuit board from one place to another. A requirement for elimination of disturbances in data transmission by wireless communication devices is also the interference shielding capacity of transmission lines and, particularly in the case of microwave-frequency signals, also the shielding of other components, such as those placed on a circuit board, from the electromagnetic field of the signal. The signal to be transferred in the transmission line can also itself induce a disturbing electromagnetic field. By using a coaxial conductor and a coaxial conductor in the transmission of the signal, it is also possible to shield other components and conductors from disturbances caused by the signal.
In a coaxial conductor, the electric circuit for transferring a signal from one place to another consists of an inner conductor and an outer conductor enclosing the inner conductor, wherein these are placed coaxially. The inner conductor is usually a wire with a circular cross-section. The outer conductor is usually cylindrical, and there is a dielectric between the inner conductor and the outer conductor. The outer conductor is usually coupled to the ground potential, and the function of the outer conductor is to provide the coaxial conductor with sufficient electromagnetic shielding, wherein the best result is achieved with a continuous and rigid tubular structure. The inner conductor and the outer conductor are made of a conductive material, usually copper. The dielectric filling the open space between the conductors functions as a mechanical support of the conductors, and it also contributes to the electric properties of the coaxial conductor, such as attenuation of the signal to be transferred. In the manufacture of coaxial cables, the outer conductor is further coated with a jacket which is usually made of a polymeric material and serves the purposes of functioning as a dielectric and protecting the coaxial conductor from mechanical wearing and environmental conditions. In coaxial cables, also twisted band layers or stranded wires are used as the outer conductor, wherein the cable can be easily bent.
Thanks to its excellent properties, the coaxial cable is widely used, but several manual work stages must be conducted when connecting the coaxial cable to a circuit board. The work stages may include soldering of the ends of the coaxial cable onto the surface or connectors of the circuit board, which will require more and more precision and time, particularly in view of the circuit boards and the components to be placed on the circuit board becoming smaller. Smaller circuit boards will also require precise placement of the coaxial cable, wherein the cable must also be bent, if necessary. However, a minimum to the radius at bend of the coaxial cable will be set by possible damage of the outer conductor, wherein the coaxial cable will emit at the damaged portion and thus cause disturbances. Small bending radii will damage also the inner conductor and the dielectric, changing the electric properties of the coaxial cable. Due to its size and large bending radii, usually about 5 to 8 times the outer diameter of the coaxial cable, coaxial cables require a large space on the circuit board.
In the manufacture of mobile phones, the different components and coaxial cables are fixed on the circuit board by means of a soldering paste, and soldering is carried out first by heating the circuit board in an oven, e.g. at 270 degrees, wherein the soldering paste melts. After this, the circuit board is cooled down, wherein the final solid soldering joints are made. A considerable disadvantage, however, is the fact that it is also possible that in the oven, the coaxial cable is wholly or partly released from the soldering paste, due to the different curling directions of the coaxial cable and the circuit board when their material is heated. Defective products increase the manufacturing costs or malfunction of the products during their use.
A known method for manufacturing a coaxial conductor on a circuit board is disclosed in the patent publication SE 462 194. The principle of the invention presented in the publication is that a long groove is cut through at least two board layers on a circuit board, particularly a multi-layer circuit board, the groove extending from a strip conductor functioning as a first ground potential to a strip conductor functioning as a second ground potential. In the next step, the groove is filled to establish a contact between the strip conductors and to build an outer conductor. After this, the circuit board is compressed in a press to make the material used in the filling to spread in the groove.
According to the publication SE 462 194, the cutting and compression of the circuit board are conducted in separate operations and phases, which, however, increases considerably the time consumed in the manufacture and thus also the costs of the circuit board. A further problem is that the precise control of the cutting depth is very difficult, because the thickness of the strip conductors can be as small as 17 micrometers. Furthermore, the cutting is complicated by the fact that the circuit board must be positioned very carefully and without clearances to avoid lateral displacement. Moreover, the thickness of different board layers can vary in different circuit boards due to manufacturing techniques, so that it is very difficult to control the cutting depth. In addition to this, one should note that when the knife mentioned in the publication is used for cutting, the circuit board is subjected to considerable forces and its damage is very probable with normal circuit board materials.
Patent publication U.S. Pat. No. 4,673,904 discloses a method for manufacturing a coaxial conductor by superimposing on a circuit board. According to the publication, the outer conductor and the inner conductor, as well as the dielectric therebetween, are formed by superimposing on a board consisting of a copper layer and a dielectric. Because of its expensiveness, the method presented is only suitable for special uses, because it is very difficult to spread, smooth and control the thickness of the dielectric material placed between the conductors and to be formed outside the coaxial conductor, which increases considerably the work stages needed in the process and the materials to be handled. The method is not suitable for use on circuit boards which comprise also other wiring, because thus said wirings and particularly the dielectric layer of the circuit board must be manufactured in the same way as the coaxial conductor itself. The result is a considerable extension in the duration of the manufacture of the circuit board. Furthermore, the way of forming the dielectric layer differs to a great extent from the conventional technique of manufacturing circuit boards.
Furthermore, it should be noted that the size of the coaxial conductor of the publication U.S. Pat. No. 4,673,904 is limited significantly by the thickness of the conductor to be superimposed and also the size of other structures of the circuit board, because the number of layers, and simultaneously also the manufacturing time, increases with the increasing size of the conductor. For this reason, the circuit board should be made thin, whereby its strength is not sufficient e.g. for supporting components and preventing buckling. Typically, circuit board layers have a thickness of 100-150 xcexcm. Moreover, it is obvious that with an increasing number of layers and a decreasing size of the conductor, the positioning of the circuit board must be conducted particularly accurately and carefully, which increases further the manufacturing costs and time.
It is a purpose of this invention to eliminate the above-mentioned drawbacks and particularly to present a structure for a coaxial conductor and a method for manufacturing a coaxial conductor particularly on a multi-layer circuit board. The invention is based on the idea that the coaxial conductor is formed in connection with the manufacture of other wirings and vias on the circuit board, preferably simultaneously. Furthermore, the invention is based on the idea that the coaxial conductor can be formed by applying simple vias and strip conductors which are known as such. Moreover, the invention is based on the idea that the structural elements required for manufacturing the coaxial conductor are formed in different board layers of the circuit board, and that the board layers are subsequently combined to form the coaxial conductor and, at the same time, the entire circuit board.
The most significant advantage of the invention is that the coaxial conductor can be formed in the board layers of the circuit board simultaneously with other wirings. Finally, the coaxial conductor is composed when the different board layers of the multi-layer circuit board are connected e.g. by pressing to form the entire circuit board. Thus, the manufacture of the conductor requires no separate work stages, equipment or tools. For manufacturing the conductor, use is made of vias and strip conductors embedded in different layers, these being typical structures to be placed on a circuit board; consequently, the technique is inexpensive and fast. In the simplest way, the vias of the coaxial conductor can be formed by borings which are either filled in or coated. Another advantage of the invention is that by the technique of the invention, it is possible to construct large conductors, having a thick dielectric layer, in a fast and simple way.
A remarkable advantage of the coaxial conductor of the invention is that it can be integrated on a multi-layer circuit board, wherein the coaxial conductor can be manufactured in connection with the manufacture of other conductor patterns of the circuit board. Thus, it is possible to avoid a separate installation of the coaxial conductor on the circuit board. Because the coaxial conductor can be integrated in the multi-layer circuit board, space is simultaneously released for components to be installed on the surface of the circuit board. By means of the invention, it is possible to eliminate problems arising e.g. in the manufacture of mobile phones, and the reliability of the products is improved. By means of the invention, the coupling of the coaxial conductor and the conductor patterns placed between the board layers of the multi-layer circuit board to each other will be very simple, wherein the number of solderings required can be reduced. A remarkable advantage is also the fact that changes in the direction of the coaxial conductor of the invention can be made without limits set by bending radii. Further, branching of the coaxial conductor can be implemented in a very simple manner, avoiding the need for separate connectors and manual work stages. As a result, the manufacturing costs of a device comprising a coaxial conductor according to the invention are reduced in comparison with those of prior art.